1. Field of the Invention
The present invention relates to a surface texture measuring instrument.
Specifically, it relates to, for instance, a surface scanning measuring instrument that scans an object surface to measure a profile, surface roughness, waviness and the like of an object to be measured.
2. Description of Related Art
A surface scanning measuring instrument that scans an object surface to measure a three-dimensional profile of an object has been traditionally known.
FIG. 20 shows an arrangement of a measurement system 100 as an example of a surface scanning measuring instrument that utilizes a scanning probe 130.
The measurement system 100 includes: a coordinate measuring machine (CMM) 110 that moves a scanning probe 130; a console 150 for manually operating the CMM 110; a motion controller 160 for controlling the operation of the CMM 110; and a host computer 200 that operates the CMM 110 through the motion controller 160 and processes the measurement data obtained by the CMM 110 to calculate the dimension and profile of the object W.
The CMM 110 includes: a table 111; a drive mechanism 120 vertically provided on the table 111 to three-dimensionally move the scanning probe 130; and a drive sensor (not shown) for detecting the drive amount of the drive mechanism 120.
The drive mechanism 120 includes: two beam supports 121 extending in Zm-axis direction (substantially vertical direction) from both sides of the table 111, the beam supports being capable of slide movement in Ym-axis direction (i.e. along the sides of the table 111); a beam 122 supported on upper ends of the beam supports 121, the beam extending in Xm-axis direction; a column 123 provided on the beam 122 in a manner slidable along the Xm-axis direction and having a guide in the Zm-axis direction; and a spindle 124 slidable in the column 123 in the Zm-axis direction, the spindle holding the scanning probe 130 at the lower end thereof.
The drive sensor includes a Ym axis sensor that detects the movement of the beam supports 121 in the Ym-axis direction, an Xm axis sensor that detects the movement of the column 123 in the Xm-axis direction and a Zm axis sensor that detects the movement of the spindle 124 in the Zm-axis direction.
As shown in FIG. 21, the scanning probe 130 includes a stylus 131 including a contact piece (measurement piece) 132 at a distal end thereof and a holder 133 that holds a base end of the stylus 131 in a manner slidable in Xp, Yp and Zp directions within a predetermined limit.
The holder 133 includes a slide mechanism (not shown) that has an xp slider, yp slider and zp slider capable of movement in mutually orthogonal directions and a probe sensor (not shown) that detects the displacement of the slide mechanism in the respective axis directions and outputs the detected displacement.
The stylus 131 is supported by a slide mechanism in a manner capable of slide movement relative to the holder 133 within a predetermined limit.
Incidentally, such an arrangement of the scanning probe 130 is disclosed in, for instance, Document 1 (JP-A-05-256640).
In thus arranged instrument, scanning movement of the scanning probe 130 is conducted along the object surface S while the contact piece 132 is in contact with the object surface S with a reference retraction Δr.
At this time, the movement locus of the scanning probe 130 can be obtained based on the drive amount of the drive mechanism 120.
It should be noted that, though the movement locus of the scanning probe 130 represents a movement locus of the contact piece 132, the contact point of the contact piece 132 against the object surface S resides at a position being offset by a predetermined distance (ΔQ) relative to the movement locus of the center of the contact piece 132.
Accordingly, after obtaining the position of the contact piece 132 by summing the position of the scanning probe 130 detected by the drive sensor and the displacement of the stylus 131 detected by the probe sensor, the position of the contact piece 132 is corrected for the predetermined offset value (ΔQ) to calculate the position of the object surface S.
At this time, it should not be neglected that inertia force is applied on a component that is driven with acceleration when the object surface S is to be scanned by a surface scanning measuring instrument.
For instance, when the object W shows a circular or arc profile, centrifugal force is generated on account of circular motion, which causes deformation of the drive mechanism 120 (the spindle 124) as shown in FIG. 22.
When such deformation is induced by acceleration, the value detected by the drive sensor contains an error corresponding to the deformation.
For instance, when the centrifugal force is generated, the detection value of the drive sensor is biased toward an interior of a circle in accordance with outward deformation of the spindle 124, which manifest itself as, for instance, a radial displacement as shown in FIG. 23.
Incidentally, L1 represents a diameter of a ring gauge and L2 represents a measurement data in FIG. 23.
Such a problem is very prominently recognized when a high-speed scanning measurement has to be conducted with a large-size CMM 110 for measuring, for instance, a body of a vehicle.
In this regard, for instance, Document 2 (JP-A-07-324928) discloses the following method for correcting a measurement error caused by acceleration.
Specifically, a correction value representing deformation characteristics of a measurement slider is obtained in advance in the form of a function of the position and acceleration of the measurement slider.
The function of acceleration and deformation characteristics can be obtained by, for instance, measuring a diameter-known ring gauge at a variety of levels of acceleration and various positions within a measurement area.
When an object is measured, in addition to obtaining a detection data by sensors, a correction value is identified based on acceleration during the measurement to correct the detection data with the correction value.
A correct measurement value is thus obtained by canceling a measurement error generated by acceleration.
In order to obtain the acceleration during measurement, the Document 2 discloses: twice differentiating the measurement value of the position of the measurement slide (paragraph 0037, claim 12); and detecting the acceleration of the measurement slider by providing acceleration sensors (paragraph 0047, claim 13).
Though the acceleration is identified by twice differentiating the position of the measurement slider according to one of the methods of the Document 2, the resolution of the acceleration is deteriorated in reverse proportion to square of sampling frequency when the position detection value is differentiated twice.
For instance, when the sampling frequency is increased by ten times, the resolution of the acceleration to be detected is deteriorated to one hundredth. Consequently, the resolution of the correction amount is also deteriorated to one hundredth.
As discussed above, it is not practical to acquire the acceleration by differentiating the position twice and is insufficient to meet the demand for high-speed and highly accurate measurement.
Further, though the acceleration of the measurement slider is obtained by twice differentiating the position of the measurement slider, what should be considered during the actual measurement is acceleration and deformation on the distal end or probe of the spindle 124. Accordingly, accuracy of correction is limited in principle when the acceleration is obtained on the measurement slider.
Incidentally, though the Document 2 also shows acquisition of the acceleration of the measurement slider by providing acceleration sensors, no required performance and installation method are not mentioned therein, which proves that the method is practically inapplicable.
For instance, considering actual measurement of acceleration by an acceleration sensor, when a circle of 100 mm diameter is scanned at a scanning velocity of 10 mm/sec, approximately 50 μG centripetal acceleration is generated. However, it is difficult to provide an acceleration sensor capable of detecting 50 μG acceleration for each of movement axes (totaling three).
Furthermore, it is impossible to provide such acceleration sensors around the probe unit.
Accordingly, it is not possible for a traditional arrangement to accurately measure a deformation caused during scanning measurement and such deformation cannot be compensated.
Thus, the profile of the object cannot be accurately obtained during high-speed measurement, so that the measurement speed is limited to a level that causes no deformation in order to conduct an accurate measurement.
Especially, since there has been a growing demand for a large-size CMM for measuring a large-size object at a high speed, a solution for the above problem has been eagerly desired.
An object of the present invention is to provide a surface texture measuring instrument capable of overcoming the traditional problem and conducting a high-speed and high-accuracy scanning measurement.